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Transcript of Network Layer (part2)6-1 INT-201: Computer Network and Communication System อ. ดร....
Network Layer (part2) 6-1
INT-201:Computer Network and
Communication System
อ.ดร . ภั�ทร ลีลีาพฤทธิ์��Dr. Pattara LeelapruteComputer Engineering DepartmentKasetsart [email protected]://www.cpe.ku.ac.th/~pattara/int201
Computer Networking: A Top Down Approach ,4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
Module6: Network Layer (part2)
Network Layer (part2) 6-2
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-3
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing, forwarding
Network Layer (part2) 6-4
u
yx
wv
z2
2
13
1
1
2
53
5
Graph: G = (N,E)
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Graph abstraction
Remark: Graph abstraction is useful in other network contexts
Example: P2P, where N is set of peers and E is set of TCP connections
Network Layer (part2) 6-5
Graph abstraction: costs
u
yx
wv
z2
2
13
1
1
2
53
5 • c(x,x’) = cost of link (x,x’)
- e.g., c(w,z) = 5
• cost could always be 1, or inversely related to bandwidth,or inversely related to congestion
Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
Question: What’s the least-cost path between u and z ?
Routing algorithm: algorithm that finds least-cost path
Network Layer (part2) 6-6
Routing Algorithm classification
Global or decentralized information?
Global: all routers have complete
topology, link cost info “link state” algorithmsDecentralized: router knows physically-
connected neighbors, link costs to neighbors
iterative process of computation, exchange of info with neighbors
“distance vector” algorithms
Static or dynamic?Static: routes change slowly
over timeDynamic: routes change more
quickly periodic update in response to link
cost changes
Network Layer (part2) 6-7
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-8
A Link-State Routing Algorithm
Dijkstra’s algorithm net topology, link costs
known to all nodes accomplished via “link
state broadcast” all nodes have same
info computes least cost paths
from one node (‘source”) to all other nodes gives forwarding table
for that node iterative: after k iterations,
know least cost path to k dest.’s
Notation: c(x,y): link cost from
node x to y; = ∞ if not direct neighbors
D(v): current value of cost of path from source to dest. v
p(v): predecessor node along path from source to v
N': set of nodes whose least cost path definitively known
Network Layer (part2) 6-9
Dijsktra’s Algorithm
1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'
Network Layer (part2) 6-10
Dijkstra’s algorithm: example
Step012345
N'u
uxuxy
uxyvuxyvw
uxyvwz
D(v),p(v)2,u2,u2,u
D(w),p(w)5,u4,x3,y3,y
D(x),p(x)1,u
D(y),p(y)∞
2,x
D(z),p(z)∞ ∞
4,y4,y4,y
u
yx
wv
z2
2
13
1
1
2
53
5
Network Layer (part2) 6-11
Dijkstra’s algorithm: example (2)
u
yx
wv
z
Resulting shortest-path tree from u:
vx
y
w
z
(u,v)(u,x)
(u,x)
(u,x)
(u,x)
destination link
Resulting forwarding table in u:
Network Layer (part2) 6-12
Dijkstra’s algorithm, discussion
Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn)
Oscillations possible: e.g., link cost = amount of carried traffic
A
D
C
B1 1+e
e0
e
1 1
0 0
A
D
C
B2+e 0
001+e1
A
D
C
B0 2+e
1+e10 0
A
D
C
B2+e 0
e01+e1
initially… recompute
routing… recompute … recompute
Network Layer (part2) 6-13
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-14
Distance Vector Algorithm
Bellman-Ford Equation (dynamic programming)
Definedx(y) := cost of least-cost path from x to y
Then
dx(y) = min {c(x,v) + dv(y) }
where min is taken over all neighbors v of x
v
Network Layer (part2) 6-15
Bellman-Ford example
u
yx
wv
z2
2
13
1
1
2
53
5Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3
du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4
Node that achieves minimum is nexthop in shortest path ➜ forwarding table
B-F equation says:
Network Layer (part2) 6-16
Bellman-Ford example 2
u
yx
wv2
2
13
1
1
3
5Clearly, dv(w) = 3, dx(w) = 2
du(z) = min { c(u,v) + dv(w), c(u,x) + dx(w),} = min {2 + 3, 1 + 2} = 3
Node that achieves minimum is nexthop in shortest path ➜ forwarding table
B-F equation says:
Network Layer (part2) 6-17
Distance Vector Algorithm
Dx(y) = estimate of least cost from x to y
Node x knows cost to each neighbor v: c(x,v)
Node x maintains distance vector Dx = [Dx(y): y є N ]
Node x also maintains its neighbors’ distance vectors For each neighbor v, x maintains
Dv = [Dv(y): y є N ]
Network Layer (part2) 6-18
Distance vector algorithm (4)
Basic idea: Each node periodically sends its own distance
vector estimate to neighbors When a node x receives new DV estimate from
neighbor, it updates its own DV using B-F equation:
Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N
Under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)
Network Layer (part2) 6-19
Distance Vector Algorithm (5)
Iterative, asynchronous: each local iteration caused by:
local link cost change DV update message from
neighbor
Distributed: each node notifies
neighbors only when its DV changes neighbors then notify
their neighbors if necessary
wait for (change in local link cost or msg from neighbor)
recompute estimates
if DV to any dest has
changed, notify neighbors
Each node:
Network Layer (part2) 6-20
x y z
xyz
0 2 7
∞ ∞ ∞∞ ∞ ∞
from
cost to
from
from
x y z
xyz
0
from
cost to
x y z
xyz
∞ ∞
∞ ∞ ∞
cost to
x y z
xyz
∞ ∞ ∞7 1 0
cost to
∞2 0 1
∞ ∞ ∞
2 0 17 1 0
time
x z12
7
y
node x table
node y table
node z table
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
32
Network Layer (part2) 6-21
x y z
xyz
0 2 7
∞ ∞ ∞∞ ∞ ∞
from
cost to
from
from
x y z
xyz
0 2 3
from
cost tox y z
xyz
0 2 3
from
cost to
x y z
xyz
∞ ∞
∞ ∞ ∞
cost tox y z
xyz
0 2 7
from
cost to
x y z
xyz
0 2 3
from
cost to
x y z
xyz
0 2 3
from
cost tox y z
xyz
0 2 7
from
cost to
x y z
xyz
∞ ∞ ∞7 1 0
cost to
∞2 0 1
∞ ∞ ∞
2 0 17 1 0
2 0 17 1 0
2 0 13 1 0
2 0 13 1 0
2 0 1
3 1 0
2 0 1
3 1 0
time
x z12
7
y
node x table
node y table
node z table
Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2
Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3
Network Layer (part2) 6-22
Distance Vector: link cost changes
Link cost changes: node detects local link cost
change updates routing info, recalculates
distance vector if DV changes, notify neighbors
“goodnews travelsfast”
x z14
50
y1
At time t0, y detects the link-cost change, updates its DV, and informs its neighbors.
At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV.
At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z.
Network Layer (part2) 6-23
Distance Vector: link cost changesLink cost changes: good news travels fast bad news travels slow - “count to infinity” problem! 44 iterations before algorithm stabilizes: see text
Poisoned reverse: If Z routes through Y to get to X :
Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)
will this completely solve count to infinity problem?
x z14
50
y60
Network Layer (part2) 6-24
Comparison of LS and DV algorithms
Message complexity LS: with n nodes, E links,
O(nE) msgs sent DV: exchange between
neighbors only convergence time varies
Speed of Convergence LS: O(n2) algorithm requires
O(nE) msgs may have oscillations
DV: convergence time varies may be routing loops count-to-infinity problem
Robustness: what happens if router malfunctions?
LS: node can advertise
incorrect link cost each node computes only
its own table
DV: DV node can advertise
incorrect path cost each node’s table used by
others • error propagate thru
network
Network Layer (part2) 6-25
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-26
Hierarchical Routing
scale: with 200 million destinations:
can’t store all dest’s in routing tables!
routing table exchange would swamp links!
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
Our routing study thus far - idealization all routers identical network “flat”… not true in practice
Network Layer (part2) 6-27
Hierarchical Routing
aggregate routers into regions, “autonomous systems” (AS)
routers in same AS run same routing protocol “intra-AS” routing
protocol routers in different AS
can run different intra-AS routing protocol
Gateway router Direct link to router
in another AS
Network Layer (part2) 6-28
3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
Intra-ASRouting algorithm
Inter-ASRouting algorithm
Forwardingtable
3c
Interconnected ASes
forwarding table configured by both intra- and inter-AS routing algorithm intra-AS sets entries
for internal dests inter-AS & Intra-As
sets entries for external dests
Network Layer (part2) 6-29
3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3c
Inter-AS tasks suppose router in AS1
receives datagram dest outside of AS1 router should
forward packet to gateway router, but which one?
AS1 must:1. learn which dests
reachable through AS2, which through AS3
2. propagate this reachability info to all routers in AS1
Job of inter-AS routing!
Network Layer (part2) 6-30
Example: Setting forwarding table in router 1d
suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c) but not via AS2.
inter-AS protocol propagates reachability info to all internal routers.
router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c. installs forwarding table entry (x,I)
3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3c
x…
Network Layer (part2) 6-31
Example: Choosing among multiple ASes
now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. this is also job of inter-AS routing protocol!
3b
1d
3a
1c2aAS3
AS1
AS21a
2c2b
1b
3cx… …
Network Layer (part2) 6-32
Learn from inter-AS protocol that subnet x is reachable via multiple gateways
Use routing infofrom intra-AS
protocol to determine
costs of least-cost paths to each
of the gateways
Hot potato routing:Choose the
gatewaythat has the
smallest least cost
Determine fromforwarding table the interface I that leads
to least-cost gateway. Enter (x,I) in
forwarding table
Example: Choosing among multiple ASes
now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x. this is also job of inter-AS routing protocol!
hot potato routing: send packet towards closest of two routers.
Network Layer (part2) 6-33
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-34
Intra-AS Routing
also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols:
RIP: Routing Information Protocol
OSPF: Open Shortest Path First
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)
Network Layer (part2) 6-35
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-36
RIP ( Routing Information Protocol)
distance vector algorithm included in BSD-UNIX Distribution in 1982 distance metric: # of hops (max = 15 hops)
DC
BA
u v
w
x
yz
destination hops u 1 v 2 w 2 x 3 y 3 z 2
From router A to subsets:
Network Layer (part2) 6-37
RIP advertisements
distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)
ech advertisement: list of up to 25 destination nets within AS
Network Layer (part2) 6-38
RIP: Example
Destination Network Next Router Num. of hops to dest. w A 2
y B 2 z B 7
x -- 1…. …. ....
w x y
z
A
C
D B
Routing table in D
Network Layer (part2) 6-39
RIP: Example
Destination Network Next Router Num. of hops to dest. w A 2
y B 2 z B A 7 5
x -- 1…. …. ....Routing table in D
w x y
z
A
C
D B
Dest Next hops w - 1 x - 1 z C 4 …. … ...
Advertisementfrom A to D
Network Layer (part2) 6-40
RIP: Link Failure and Recovery If no advertisement heard after 180 sec -->
neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements
(if tables changed) link failure info quickly (?) propagates to entire
net poison reverse used to prevent ping-pong
loops (infinite distance = 16 hops)
Network Layer (part2) 6-41
RIP Table processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated
physical
link
network forwarding (IP) table
Transprt (UDP)
routed
physical
link
network (IP)
Transprt (UDP)
routed
forwardingtable
Network Layer (part2) 6-42
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-43
OSPF (Open Shortest Path First)
“open”: publicly available uses Link State algorithm
LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm
OSPF advertisement carries one entry per neighbor router
advertisements disseminated to entire AS (via flooding) carried in OSPF messages directly over IP (rather than
TCP or UDP
Network Layer (part2) 6-44
OSPF “advanced” features (not in RIP)
security: all OSPF messages authenticated (to prevent malicious intrusion)
multiple same-cost paths allowed (only one path in RIP)
For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time)
integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology
data base as OSPF hierarchical OSPF in large domains.
Network Layer (part2) 6-45
Hierarchical OSPF
Network Layer (part2) 6-46
Hierarchical OSPF
two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas. area border routers: “summarize” distances to nets
in own area, advertise to other Area Border routers. backbone routers: run OSPF routing limited to
backbone. boundary routers: connect to other AS’s.
Network Layer (part2) 6-47
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-48
Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard
BGP provides each AS a means to:1. Obtain subnet reachability information from
neighboring ASs.2. Propagate reachability information to all AS-
internal routers.3. Determine “good” routes to subnets based
on reachability information and policy. allows subnet to advertise its existence
to rest of Internet: “I am here”
Network Layer (part2) 6-49
BGP basics pairs of routers (BGP peers) exchange routing info
over semi-permanent TCP connections: BGP sessions BGP sessions need not correspond to physical links.
when AS2 advertises prefix to AS1: AS2 promises it will forward any addresses
datagrams towards that prefix. AS2 can aggregate prefixes in its advertisement
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
Network Layer (part2) 6-50
Distributing reachability info using eBGP session between 3a and 1c, AS3 sends prefix
reachability info to AS1. 1c can then use iBGP do distribute new prefix info to all
routers in AS1 1b can then re-advertise new reachability info to AS2
over 1b-to-2a eBGP session when router learns of new prefix, creates entry for prefix
in its forwarding table.
3b
1d
3a
1c2aAS3
AS1
AS21a
2c
2b
1b
3ceBGP session
iBGP session
Network Layer (part2) 6-51
Path attributes & BGP routes
advertised prefix includes BGP attributes. prefix + attributes = “route”
two important attributes: AS-PATH: contains ASs through which prefix
advertisement has passed: e.g, AS 67, AS 17 NEXT-HOP: indicates specific internal-AS router
to next-hop AS. (may be multiple links from current AS to next-hop-AS)
when gateway router receives route advertisement, uses import policy to accept/decline.
Network Layer (part2) 6-52
BGP route selection
router may learn about more than 1 route to some prefix. Router must select route.
elimination rules:1. local preference value attribute: policy
decision2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato
routing4. additional criteria
Network Layer (part2) 6-53
BGP messages
BGP messages exchanged using TCP. BGP messages:
OPEN: opens TCP connection to peer and authenticates sender
UPDATE: advertises new path (or withdraws old)
KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request
NOTIFICATION: reports errors in previous msg; also used to close connection
Network Layer (part2) 6-54
BGP routing policy
A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks
X does not want to route from B via X to C .. so X will not advertise to B a route to C
A
B
C
W X
Y
legend:
customer network:
provider network
Network Layer (part2) 6-55
BGP routing policy (2)
A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C?
No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers
B wants to force C to route to w via A B wants to route only to/from its customers!
A
B
C
W X
Y
legend:
customer network:
provider network
Network Layer (part2) 6-56
Why different Intra- and Inter-AS routing ?
Policy: Inter-AS: admin wants control over how its traffic
routed, who routes through its net. Intra-AS: single admin, so no policy decisions
needed
Scale: hierarchical routing saves table size, reduced
update trafficPerformance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance
Network Layer (part2) 6-57
Chapter 4: Network Layer
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer (part2) 6-58
R1
R2
R3 R4
sourceduplication
R1
R2
R3 R4
in-networkduplication
duplicatecreation/transmissionduplicate
duplicate
Broadcast Routing deliver packets from source to all other nodes source duplication is inefficient:
source duplication: how does source determine recipient addresses?
Network Layer (part2) 6-59
In-network duplication
flooding: when node receives brdcst pckt, sends copy to all neighbors Problems: cycles & broadcast storm
controlled flooding: node only brdcsts pkt if it hasn’t brdcst same packet before Node keeps track of pckt ids already brdcsted Or reverse path forwarding (RPF): only forward
pckt if it arrived on shortest path between node and source
spanning tree No redundant packets received by any node
Network Layer (part2) 6-60
A
B
G
DE
c
F
A
B
G
DE
c
F
(a) Broadcast initiated at A (b) Broadcast initiated at D
Spanning Tree
First construct a spanning tree Nodes forward copies only along
spanning tree
Network Layer (part2) 6-61
A
B
G
DE
c
F1
2
3
4
5
(a) Stepwise construction of spanning tree
A
B
G
DE
c
F
(b) Constructed spanning tree
Spanning Tree: Creation Center node Each node sends unicast join message to
center node Message forwarded until it arrives at a node already
belonging to spanning tree
Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting
routers having local mcast group members tree: not all paths between routers used source-based: different tree from each sender to rcvrs shared-tree: same tree used by all group members
Shared tree Source-based trees
Approaches for building mcast treesApproaches: source-based tree: one tree per source
shortest path trees reverse path forwarding
group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees
…we first look at basic approaches, then specific protocols adopting these approaches
Shortest Path Tree
mcast forwarding tree: tree of shortest path routes from source to all receivers Dijkstra’s algorithm
R1
R2
R3
R4
R5
R6 R7
21
6
3 4
5
i
router with attachedgroup member
router with no attachedgroup member
link used for forwarding,i indicates order linkadded by algorithm
LEGENDS: source
Reverse Path Forwarding
if (mcast datagram received on incoming link on shortest path back to center)
then flood datagram onto all outgoing links else ignore datagram
rely on router’s knowledge of unicast shortest path from it to sender
each router has simple forwarding behavior:
Reverse Path Forwarding: example
• result is a source-specific reverse SPT– may be a bad choice with asymmetric links
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
datagram will be forwarded
LEGENDS: source
datagram will not be forwarded
Reverse Path Forwarding: pruning forwarding tree contains subtrees with no mcast
group members no need to forward datagrams down subtree “prune” msgs sent upstream by router with
no downstream group members
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
prune message
LEGENDS: source
links with multicastforwarding
P
P
P
Shared-Tree: Steiner Tree
Steiner Tree: minimum cost tree connecting all routers with attached group members
problem is NP-complete excellent heuristics exists not used in practice:
computational complexity information about entire network needed monolithic: rerun whenever a router needs
to join/leave
Center-based trees
single delivery tree shared by all one router identified as “center” of tree to join:
edge router sends unicast join-msg addressed to center router
join-msg “processed” by intermediate routers and forwarded towards center
join-msg either hits existing tree branch for this center, or arrives at center
path taken by join-msg becomes new branch of tree for this router
Center-based trees: an example
Suppose R6 chosen as center:
R1
R2
R3
R4
R5
R6 R7
router with attachedgroup member
router with no attachedgroup member
path order in which join messages generated
LEGEND
21
3
1
Internet Multicasting Routing: DVMRP
DVMRP: distance vector multicast routing protocol, RFC1075
flood and prune: reverse path forwarding, source-based tree RPF tree based on DVMRP’s own routing tables
constructed by communicating DVMRP routers no assumptions about underlying unicast initial datagram to mcast group flooded
everywhere via RPF routers not wanting group: send upstream
prune msgs
DVMRP: continued…
soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned: mcast data again flows down unpruned branch downstream router: reprune or else continue
to receive data routers can quickly regraft to tree
following IGMP join at leaf odds and ends
commonly implemented in commercial routers Mbone routing done using DVMRP
Tunneling
Q: How to connect “islands” of multicast routers in a “sea” of unicast routers?
mcast datagram encapsulated inside “normal” (non-multicast-addressed) datagram
normal IP datagram sent thru “tunnel” via regular IP unicast to receiving mcast router
receiving mcast router unencapsulates to get mcast datagram
physical topology logical topology
PIM: Protocol Independent Multicast
not dependent on any specific underlying unicast routing algorithm (works with all)
two different multicast distribution scenarios :
Dense: group members
densely packed, in “close” proximity.
bandwidth more plentiful
Sparse: # networks with group
members small wrt # interconnected networks
group members “widely dispersed”
bandwidth not plentiful
Consequences of Sparse-Dense Dichotomy: Dense group membership by
routers assumed until routers explicitly prune
data-driven construction on mcast tree (e.g., RPF)
bandwidth and non-group-router processing profligate
Sparse: no membership until
routers explicitly join receiver- driven
construction of mcast tree (e.g., center-based)
bandwidth and non-group-router processing conservative
PIM- Dense Mode
flood-and-prune RPF, similar to DVMRP but
underlying unicast protocol provides RPF info for incoming datagram
less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm
has protocol mechanism for router to detect it is a leaf-node router
PIM - Sparse Mode
center-based approach router sends join msg
to rendezvous point (RP) intermediate routers
update state and forward join
after joining via RP, router can switch to source-specific tree increased performance:
less concentration, shorter paths
R1
R2
R3
R4
R5
R6R7
join
join
join
all data multicastfrom rendezvouspoint
rendezvouspoint
PIM - Sparse Mode
sender(s): unicast data to RP,
which distributes down RP-rooted tree
RP can extend mcast tree upstream to source
RP can send stop msg if no attached receivers “no one is listening!”
R1
R2
R3
R4
R5
R6R7
join
join
join
all data multicastfrom rendezvouspoint
rendezvouspoint
Network Layer (part2) 6-79
Chapter 4: summary
4. 1 Introduction 4.2 Virtual circuit
and datagram networks
4.3 What’s inside a router
4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing